1. Field of the Invention
This invention relates to a process for extracting hydrocarbons from the earth. More particularly, this invention relates to a method for recovering viscous hydrocarbons such as bitumen from a subterranean reservoir by injecting a heated fluid via a well into the reservoir to lower the viscosity of the viscous hydrocarbons and to create fluid mobility, and by injecting a hydrocarbon solvent to assist in recovery of the viscous hydrocarbons.
2. Description of the Prior Art
In many areas of the world, there are larger deposits of viscous petroleum, such as the Athabasca and Cold Lake region in Alberta, the Jobo region in Venezuela, and the Edna and Sisquoc regions in California, U.S.A. These deposits are often referred to as "tar sand" or "heavy oil" deposits due to the high viscosity of the hydrocarbons which they contain. While some distinctions have arisen between tar sands, bitumen and heavy oil, these terms will be used interchangeably herein. These tar sands may extend for many miles and occur in varying thicknesses of up to more than 300 feet. Although these deposits may lie at or near the earth's surface, generally they are located under a substantial overburden which may be as great as several thousand feet thick. Tar sands located at these depths constitute some of the world's largest presently known petroleum deposits. The tar sands contain a viscous hydrocarbon material, commonly referred to as bitumen, in an amount which ranges up to about 20% by weight. Bitumen can be considered to be effectively immobile at typical reservoir temperatures. For example, in the Cold Lake region of Alberta, at a typical reservoir temperature of about 13.degree. C. (about 55.degree. F.), bitumen is immobile with a viscosity exceeding several thousand poises. However, at higher temperatures, such as temperatures exceeding 93.degree. C. (about 200.degree. F.), the bitumen generally becomes mobile with a viscosity of less than 345 centipoises.
Since most tar sand deposits are too deep to be mined economically, various in situ recovery processes have been proposed for separating the bitumen from the sand in the formation itself and producing the bitumen through a well drilled into the deposit. Among the various methods for in situ recovery of bitumen from tar sands, processes which involve the injection of steam are usually the first to be considered for application. Steam can be utilized to heat and fluidize the immobile bitumen and, in some cases, to drive the mobilized bitumen towards production means.
The most common and proven method for recovering viscous hydrocarbons is by using a steam stimulation technique, commonly called the "huff and puff" or "steam soak" process. In this type of process, steam is injected into a formation by means of a well and the well is shut-in to permit the steam to heat the bitumen, thereby reducing its viscosity. Subsequently, all formation fluids, including mobilized bitumen, water and steam, are produced from the same well using the previously injected steam as the driving force for production. Initially, sufficient pressure may be available in the production interval to lift fluids to the surface; as the pressure falls, artificial lifting methods are normally employed. Production is terminated when no longer economical and steam is injected again. This cycle is then repeated many times until oil production is no longer economical.
During the early cycles of steam injection and production, oil production rates may be quite high since the oil nearest to the well is being produced. However, during subsequent steam cycles as the oil nearest the well is depleted, steam must move farther into the formation to contact the oil and as a result increased heat losses make the steam less effective as an oil recovery agent. The process loses efficiency and eventually oil production becomes uneconomic.
Another general method for recovering viscous hydrocarbons is by using "thermal drive" processes. Such processes employ at least two wells--an injection well and a production well, spaced apart from each other by some distance and extending into the heavy oil formation. In operation, a heated fluid (such as steam or hot water) is injected through the injection well into the formation where it mixes with the heavy oil and drives the heated fluids toward the production well. A serious problem with thermal drive processes is that the driving force of the flowing heated fluid is lost upon break through at the production well. Moreover, because of the large reservoir volume which must be treated with the heated fluid, much of the heat value dissipates uselessly into the formation and is lost.
Various methods have been proposed for improving these thermal recovery processes. Many involve the injection of a nonaqueous solvent. For example, Canadian Pat. No. 1,036,928 granted to the Dow Chemical Company on Aug. 22, 1978 discloses a process which involves injecting hot solvent vapors by themselves into a tar sand formation to recover only a portion of the oil. A very serious problem with this process it that to treat the large reservoir volumes with solvent alone would be prohibitively expensive.
Thus, others have proposed injecting solvent and steam. For example, U.S. Pat. No. 4,026,358 which issued on May 31, 1977 to Joseph C. Allen discloses a process which involves injecting a solvent followed by establishing a thermal sink in the formation by the injection of steam. Solvent is injected in this instance to improve the conformance of the thermal recovery method, i.e. to improve the horizontal and vertical sweep efficiencies. However, there is no assurance that by injecting solvent before injecting steam, the solvent will penetrate into the tar sand formation to a sufficient degree.
Yet another method disclosed in the patent literature is that disclosed in U.S. Pat. No. 4,034,812 which issued on July 12, 1977 to Richard A. Widmyer. This method involves injecting a heated fluid into the tar sand formation until the viscous petroleum is heated and physically separates in situ from unconsolidated sand. The sand then settles toward the bottom of a cavity created in the formation. Solvent is injected in order to assist in the separation of the viscous petroleum from the sand. However, those skilled in the art will recognize the difficulties of creating and sustaining an underground cavity that could be used for oil separation. If one were to establish such a cavity, problems may exist with this process in that prohibitively long periods of time may be necessary in order for the tar sands to separate. Further, during the time the bitumen is settling, heat is being dissipated and lost to the formation. The addition of a solvent prior to producing the oil is said to enhance the rate of separation of the sand from the oil.
While the above methods are of interest, the fact remains that this technology has not generally been economically attractive for commerical development of tar sands. Substantial problems exist with each process of the prior art. As mentioned, the only in situ process which has been proven to be effective commercially is the steam stimulation process and this process only recovers a small portion of the bitumen with declining effectiveness after each steam injection/production cycle. Therefore, there is a continuing need for an improved thermal process for the effective recovery of viscous hydrocarbons from subterranean formations such as tar sand deposits.